SUMMARY

Understanding the mechanisms for fatigue crack initiation and propagation in micron-scale silicon (Si) is of great importance to assess and improve the reliability of Si-based MEMS in harsh environments. Accordingly, this investigation studies the fatigue properties of 10-micron-thick single-crystal Si (SCSi) films using kHz-frequency resonating structures under fully-reversed loading. Overall, the stress plays a major role on the fatigue properties: decreasing stress amplitudes from ~3-3.5 to ~1.5-2 GPa results in an increase of 8 orders in lifetime, and a decrease in degradation rate by 4-5 orders of magnitude. In addition to stress, the influences of resonant frequency (4 vs. 40kHz) and environment (30°C,50%RH vs. 80°C,30%RH and 80°C,90%RH) on the resulting S-N curves and resonant frequency evolution are thoroughly investigated. In the high- to very high-cycle fatigue (HCF/VHCF) regime, both the frequency and environment strongly affect the fatigue properties. Damage accumulation rates are significantly higher (1 to 2 orders) in harsh environments. The separate influence of humidity, affecting the adsorbed water layer thickness, is also highlighted at 80°C: decrease rates are measured up to one order lower at 30%RH than at 90%RH. Moreover, a strong influence of frequency is detected. These observations bring further evidence supporting reaction-layer fatigue as a viable description of the HCF/VHCF behavior of micron-scale Si. Regarding the low-cycle fatigue (LCF) properties, continuous, monotonic damage accumulation, beginning after the first burst (as low as ~500 cycles), is observed. The decrease rates are not as much influenced by the environment as in HCF/VHCF, although they are higher in harsh environments. An investigation on time-dependent subcritical cracking for the LCF regime is implemented, but does not appear to fully account for the experimental results. The details of the underlying mechanism for the LCF behavior of SCSi films remain unclear.